When you "agree to disagree" it means that you don't have the same opinion, but you can accept that the other person has a different point of view. You don't have to agree on everything to continue you're discussions or relationship.
In this case, neither country is going to set up an armed outpost to defend where it thinks the border is.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.